1. Field of the Invention
The invention relates to a rotor structure and a rotor/stator structure of motors employing permanent magnets, some aspects of the invention particularly applying to a rotational motor and a linear motor.
2. Description of the Related Art
Motors employing permanent magnets, commonly called brushless motors, are widely used as servo control motors in commercial products and other devices.
FIG. 13 is a sectional view of a related permanent magnet motor. Reference numeral 1 represents a rotor axis which is also a magnetic path for the field flux. Reference numeral 2 represents a permanent magnet, and recently, cylindrical, sintered magnets made of Nd-Fe-B (Neodymium, Iron, Boron) system are often used. The permanent magnet 2 of the rotor is magnetized in 8 poles.
SC represents the yoke section for the stator and SCT represents a tooth of the stator. 24 slots are surrounded by the teeth. Around each slot is coiled 3-phase 8-pole stator winding and respective windings are labeled such as U1C, W8C, V1C, etc. FIG. 14 shows a specific example of the connection of each winding. U, V, and W are 3-phase terminals of the motor, and U1C-U2C, for example, represents a winding which is coiled between winding labels U1C and U2C in FIG. 13.
The operational principle of this permanent magnet motor is identical to the operational principle of a common brushless motor. A rotational torque is generated by applying a current through a winding with higher change rate of rotor rotation, that is, higher d.phi./d.theta., of the linkage flux of each of the windings, in response to the rotor rotation. When the rotor is rotating at a constant rate, by applying a 3-phase alternating current through each of the U, V, and W windings in synchronization with the rotor rotation, in principle, any torque of any magnitude can be obtained. The magnitude of the 3-phase alternating current is proportional to the magnitude of the desired torque.
In the permanent magnet motor shown on FIG. 13, the torque T generated by the current in each winding can be represented by EQU T=KT.multidot.I.multidot.NT.multidot.d.phi./d.theta. (1)
when considered partly. Here, KT is the torque constant, I the applied current, and NT the number of coils for the winding. d .phi./.theta. is the change rate of rotation of the flux linked to the winding, and a torque proportional to d.phi./d.theta. can be obtained. A slot of the stator has an opening. The magnetic resistance of the stator side, as seen from the rotor side, has its magnetic resistance at the slot opening large but the width of the slot opening small, so that the flux produced by the permanent magnet on the rotor surface can be considered to go through the teeth of the stator nearly evenly. Therefore, the change rate of rotation d.phi./d.theta. of the flux and the generated torque of the motor are proportional to the flux density of the permanent magnet.
An objective of the present invention is to provide a motor in which a larger torque can be generated to thereby increase motor efficiency and reduce costs, and wherein the motor torque ripple is reduced and a precise control with less vibration and noise can be achieved.
One problem with the existing art is that, because of a constraint on the flux density of the permanent magnet, the saturated flux density of about 1.8 T for flat rolled magnetic steel sheets and strip, which is a silicon steel plate, is not efficiently used.
In general, force F=(flux density B).times.(current I).times.(effective length of the acting wire), as dictated by Fleming's left-hand rule. In the stator shown on FIG. 13, the teeth of the stator for letting flux through and each of the windings coiled on the slot are distributed throughout the circumference. The flux density B and the current I are related to each other in a tradeoff relationship because of the space consideration of the stator. In general, the maximum force F occurs when the flux density B and the current I are each 50%, that is, when the magnetic loading and the electric loading are of the same order. Therefore, the average flux density at the gap of the stator and the rotor is one half (about 0.9 T) of the maximum flux density (1.8 T) of the flat rolled magnetic steel sheets and strip. When a rare earth permanent magnet is used as the permanent magnet 2, the flux density is about 1.1 T for a magnet with a large flux density, and the motor architecture has a slightly larger magnetic loading.
Therefore, the permanent magnet motor shown on FIG. 13 has a problem that the change rate of rotation, d.phi./d.theta., of the flux is not efficiently using the saturated flux density 1.8 T of the flat rolled magnetic steel sheets and strip, that is, a problem that the generated torque is small.
Moreover, the torque ripples reduction techniques used in this related art employ a combination of a method to make the flux density in the rotational direction of the rotor generated by the rotor into more of a sine wave shape by making the magnet shape on the rotor surface a hog-backed smooth convex, a method to reduce the harmonic components by making the distribution of the windings more sine wave distribution, and a method to reduce the harmonic components by skewing the rotor or the stator. All of these suffer from the problem that motor output torque is reduced.
Stator problems also are present including a problem of number of work processes involved due to the complexity of the work to coil the winding to each slot of the permanent magnet motor shown on FIG. 13, a problem of the winding density for winding of each slot being limited to only about 40%, a problem of the motor length becoming long as a result of the coil end becoming longer due to the requirement that the winding need to be inserted into the slot, and a problem of the temperature increase at the coil end section becoming larger due to the coil end section becoming larger. All of these problems add to the cost of designing and producing a motor.